The prior art related to the development of plastic window frames includes the use of metal stiffeners located within the hollow portions of the window sashes. The window sashes are usually formed of an extrudable plastic such as vinyl. Conventionally, the metal stiffeners are extruded or roll formed members which are shaped to fit the form of the hollow window sashes. The conventional metal stiffeners are shaped to conform to the walls of the hollow window sashes and comprise a relatively large portion of the overall weight of the window frame.
The disadvantages of the conventional vinyl window sashes which utilize the extruded or roll formed metal stiffeners includes the extensive heat conduction path which is provided by the walls of the metal stiffeners. This disadvantage is especially significant since it tends to degrade the thermal insulating properties of the vinyl window sash which is one of the primary advantages of this type of plastic window frame construction.
Another disadvantage of the conventional vinyl window apparatus is the complex and costly tooling required to fabricate conventional metal stiffeners which contributes toward the relatively high cost of a conventional vinyl window.